When you have the actual OCD, it's called a disorder because it often interferes with your life (such as you MUST touch all the books in a library, even when the building's on fire) or you're doing it because you feel the need and it taunts you.

OCD tendancies typically mean that you have things that you do that may seem like OCD, although you don't have the disorder. I.e., your only problem is that your pencils NEED to be very sharp or if you twitch one leg 5 times, you feel the need to twitch the other leg 5 times.

OCD tendancies appear in people with OCD, although they may be worse and they usually come with other, more worse habits.